1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser beam printer, a copying machine, a facsimile and the like and an improvement in a sheet supply portion of such an apparatus.
2. Related Background Art
Recently, laser beam printers have been made more compact and inexpensive, with the result that the laser beam printers which were used under limited conditions such as office equipment and/or business purposes have been used in private offices and/or homes since personal computers have progressed. To this end, the laser beam printers themselves have been improved so that the performance thereof is enhanced and so that they can easily be used by any person. For example, by only opening an open/close cover, the user himself can easily exchange consumed parts such as a process cartridge and easily effect jam treatment.
Further, in the past, a large number of sheets having certain characteristics recommended by electrophotographic makers were used. However, recently, various kinds of paper sheets (for example, no-brand low cost sheets, ink jet printing sheets and the like) available from stationery shops or discount stores have frequently been used. Further, for an individual, a user may use various sheets such as a post card, a letter-paper, an envelope and sheets having various thicknesses.
Since such sheets may have features different from features of sheets to be used in the apparatuses recommended by the makers, various problems may occur. For example, some kinds of paper sheets may reduce a friction force of a sheet supply roller for supplying a sheet to the laser beam printer, thereby causing poor sheet supply. More particularly, a coating on the sheet, a pigment coated on the sheet and/or additives to the sheet (talc, calcium carbonate or Kaolin) can adhere to and accumulate on a surface of the sheet supply roller, and, a rubber surface of the sheet supply roller is worn by a rough surface of the sheet or fibers of the sheet, thereby causing such problems.
Such sheet supply roller must be cleaned by using a solvent such as alcohol, or the sheet supply roller itself must be exchanged. However, since the solvent such as alcohol may not easily be available and the sheet supply roller cannot easily be exchanged by the user (even when the improvement is made so that anyone can handle the apparatus easily), generally, the user leaves repair of the apparatus to the maker.
To avoid this, it is requested to provide an arrangement in which any user can easily exchange the sheet supply roller. However, the conventional arrangements have the following disadvantages.
(1) Since the sheet supply roller is disposed within the body of the apparatus, a space available to exchange the sheet supply roller is small. PA0 (2) As shown in FIG. 7A, in general, in a sheet supply roller unit 100, a sheet supply roller 101, a sheet supply sub-roller 102 and cams 106, 109 are fitted on a sheet supply shaft 104 and are secured to the sheet supply shaft by fixing members 103, 105, 108 and 110. When parts are exchanged, the unit 100 integrally including the sheet supply shaft 104 is dismounted from a body of the apparatus, and, as shown in FIG. 7B, the sub-roller 102 and the fixing members (parallel pins) 103, 105 provided on the shaft are slid along a longitudinal direction of the shaft to be removed. Accordingly, it is difficult for the user to exchange the sheet supply roller. PA0 (3) A sheet supply roller unit 122 shown in FIG. 8 includes a rubber portion 122a having a high coefficient of friction and a sheet supply roller metal core 122b. In this unit, the user can easily exchange the sheet supply roller only by disengaging a pawl 122c of a snap fit 122d provided on the metal core 122b from a recess 120a provided in a shaft 120 and sliding the roller laterally. However, as shown in FIG. 8, the exchange of the sheet supply roller can be effected only when the sheet supply roller is supported by a bearing portion 121 in a cantilever fashion and other parts are not provided on the shaft. Other arrangements cannot be used.